Method for synchronized polarization switching of an optical transmission signal of an optical transmission system having superheterodyne reception and an apparatus for the implementation of said method

Abstract

A method for generating an FSK-modulated optical signal with an optical two-filter FSK super-heterodyne receiver. The optical signal has two different polarization conditions that are orthogonal to one another. Due to orthogonal conditions, polarization-independent reception is possible. Synchronized polarization shifting occurs to generate a polarized modulated optical signal having alternating orthogonal polarization conditions. A delay stage for delaying the switching signal by one-half a clock period ensures accurate synchronization. The frequency shift of the FSK-modulated signal can be freely selected while maintaining excellent bandwidth characteristics and optimum sensitivity.

We claim as our invention: 
     
       1. A method for synchronized switching of a polarization condition of an optical transmission signal in an optical transmitter, comprising the steps of: clocking a data signal with a defined electrical clock signal having clock pulses and a clock period at a transmitter side of said optical transmission system; said data signal having bits, and each bit having a center;   modulating an optical transmission signal using said data signal and thereby generating a modulated optical transmission signal;   delaying said electrical clock signal by one-half of said clock period and thereby generating a delayed electrical clock signal;   switching the polarization condition of said modulated optical transmission signal between orthogonal polarization, states by means of said delayed electrical clock signal; and   continuously polarization switching the polarization condition of switching said modulated optical transmission signal while being clocked by said delayed electrical clock signal to generate a switching stage output signal of two polarization conditions that are orthogonal to one another while having polarization condition switching occur in a middle of each clock period, so that the center of each of said bits is synchronized with said polarization condition switching and said middle of each clock period of the defined clock.   
     
     
       2. A transmitter for transmitting an optical transmission signal, said transmitter comprising: modulator means connected to a clock generator having a clock period and connected to a data input signal having first and second states or bits;   delay means connected to said clock generator for generating a delayed electronic signal by one-half of said clock period;   electro-optical transducer means for converting said data input signal into a polarized FSK- or ASK-modulated optical signal; and   a polarization condition switching means to which said polarized FSK-modulated or ASK-modulated optical signal and said delayed electronic signal are supplied for generating an optical transmission signal whose polarization condition is switched between orthogonal said polarization condition states, said polarization switching occurring in a middle of each clock period and synchronized with a center of each bit of said data input.   
     
     
       3. A transmitter as claimed in claim 2, wherein said polarization condition switching condition means is an integrated optical waveguide device consisting of birefringent material. 
     
     
       4. A transmitter as claimed in claim 3, wherein said waveguide consists of lithium niobate.